A steering column for a vehicle is divided into upper and lower shafts, and is constructed to be contracted in the direction of the steering shaft when a driver hits his chest, etc., on the steering wheel and an impact force is applied to his chest at the colliding time, etc., thereby relaxing the impact force.
FIG. 1 shows an attachment structure of a conventional steering column. A shaft column 2 is divided into upper and lower shafts within a jacket 1. When a steering wheel 5 is pressed in the direction of arrow A, the upper shaft compresses a buffer portion within the jacket 1, and is moved together with the jacket 1 in the direction of arrow A, thereby contracting the shaft column. The jacket 1 is supported by the bracket 3 fastened by a bolt 6 to a chassis 4.
FIGS. 2 and 3 respectively show an enlarged perspective view and a cross-sectional view of the bracket 3 and a fastening portion thereof. A notch 8 is formed in a flange portion 7 of the bracket 3, and small holes 9 for receiving shear pins are disposed around the notch 8. A capsule 10 having an H shape in cross section is fitted into the notch 8, and has a bolt hole 11 in a central portion thereof. Small holes 13 for receiving shear pins are formed in a flange portion 12 of the capsule 10. Shear pins 14 made of synthetic resin are inserted and fixed into the small holes 9 and 13.
In such a structure mentioned above, the jacket 1 is fastened by bolts to the chassis 4 through the bracket 3 and the capsule 10, thereby completely fixing the jacket 1 to the chassis. However, the bracket 3 and the capsule 10 are fixed to each other through the shear pins 14 and the fitting frictional force therebetween. Accordingly, when a strong impact force in the direction of arrow A is applied to the steering wheel 5, the shear pins 14 are broken, and the bracket 3 is detached from the capsule 10 while holding the jacket 1, thereby contracting the shaft column 2 by a contracting mechanism within the jacket and relaxing the impact force.
However, in the attachment structure of the steering column mentioned above, there are the following problems.
To facilitate the detachment of the bracket 3 at the colliding time, it is necessary to form clearance S by increasing distance L between the upper and lower flange portions 12 of the capsule 10 with respect to thickness t of the flange portion 7 of the bracket 3, so that the steering wheel is rattled, causing some problems in the operation of the vehicle.
When the capsule 10 and the bracket 3 are closely fitted to each other, the frictional force therebetween is increased, and it is thereby not easy to smoothly detach the bracket 3 from the capsule 10.
In particular, the pressing force from the steering wheel is normally applied in the horizontal direction, there causes a problem with respect to the frictional force between the upper face of the flange portion 7 of the bracket 3 and the lower face of the upper flange portion 12 of the capsule 10, in which the bracket 3 might not be detached from the capsule 10 at an emergency time since the frictional coefficient thereof is extremely high when they are made of metal such as iron, aluminum, etc.
Further, the occurrence of the collision of the vehicle is not limited on the front side thereof, but a slanting force is often applied to the vehicle. In such a case, a secondary impact force from the steering wheel has a slanting direction, causing a problem with respect to the frictional force between side faces of the notch 8 and the capsule 10.
To solve the problems mentioned above, it is considered to make the capsule of synthetic resin, but a creep is generated when the capsule made of synthetic resin is fastened by bolts, reducing the durabilitiy of the structure and causing problems in safety.